It is twenty years since NASA's DART mission collided with a satellite after depleting its fuel during a rendezvous attempt.
The Demonstration for Autonomous Rendezvous Technology (DART) mission, not to be confused with the successful Double Asteroid Redirection Test operation, which smacked into the asteroid Dimorphos in 2023, was designed to meet with a satellite in Earth orbit.
Sadly for the scientists and engineers involved, it did more than rendezvous. Due to using propellant at a greater rate than expected, the spacecraft exhausted its maneuvering fuel and ended up slowly colliding with its target before initiating a retirement burn that would eventually result in the spacecraft being safely deorbited.
The expedition, which cost $110 million (up from a proposed $47 million), was only supposed to last 24 hours. The plan was to launch it using a Pegasus rocket, fired from a carrier aircraft. DART would be placed in a parking orbit beneath its target, the MUBLCOM satellite, and fire its Hydrazine Auxiliary Propulsion System (HAPS) thrusters to position DART below and behind the target.
Initially, things appeared to be proceeding as planned. DART was 40 km behind and 7.5 km below MUBLCOM. However, controllers began to notice something was amiss with the spacecraft's navigation system. As DART entered the next phase of proximity operations to bring the spacecraft to within 200-500 m of the target, it became clear that far more maneuvering fuel was being used than anticipated. It was also clear that the mission would end prematurely since the fuel reserves would soon be exhausted.
The problem was that DART could not receive or execute uplinked commands. Controllers were powerless to stop events as they unfolded.
The increased fuel use was caused by problems in the software. Invalid navigational data resulted in continual software resets and excessive thruster firings. The problems mounted until, as the mishap report [PDF] put it: "This had the effect of lining MUBLCOM up in the 'cross hairs' of DART's guidance system at a time when the system did not have the ability to accurately control the distance between the two spacecraft."
"At the time of collision, DART was flying toward MUBLCOM at 1.5 meters per second while its navigational system thought it was 130 meters away from MUBLCOM and retreating at 0.3 meters per second."
DART did have collision avoidance software logic, but it was never designed for this scenario; engineers assumed that the navigational data could never be that inaccurate.
The collision avoidance system was eventually activated 1 minute and 23 seconds before the collision, but even then, the inaccurate perception of DART's distance from MUBLCOM and the relative velocity stopped DART from taking preventative action.
Three minutes and 49 seconds after the collision, DART initiated its retirement.
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Both spacecraft survived the closer-than-expected get together. MUBLCOM, launched for the Defense Advanced Research Projects Agency (DARPA) in 1999, had already completed its original and primary mission, and after an automatic reset, regained its operational status. The impact also pushed it into a higher orbit. The DART spacecraft re-entered the atmosphere in 2016.
Although DART might be regarded as a technological cul-de-sac, there were lessons to be learned around software validation and verification and in understanding its design flaws and extremely narrow margins for error.
The mission had been folded into the Orbital Space Plane (OSP) program. It survived the OSP cancellation because it was considered critical to in-space assembly concepts. In addition, NASA wanted its own autonomous capability for cargo delivery and International Space Station (ISS) operations.
As it turned out, the mishap report described DART as a "one-time project." While autonomous rendezvous (something other space agencies, such as Roscosmos, had been doing for decades) is essential for space exploration, DART, despite its high profile two decades ago, is now more a cautionary tale [PDF] than the technological trailblazer its designers had envisioned. ®
